Instructions

Sift the flour, cocoa and baking soda together. Set aside.
Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed until soft and creamy. Add both sugars, the salt and vanilla extract and beat for 2 minutes more.
Turn off the mixer. Pour in the dry ingredients, drape a kitchen towel over the stand mixer to protect yourself and your kitchen from flying flour and pulse the mixer at low speed about 5 times, a second or two each time. Take a peek — if there is still a lot of flour on the surface of the dough, pulse a couple of times more; if not, remove the towel. Continuing at low speed, mix for about 30 seconds more, just until the flour disappears into the dough — for the best texture, work the dough as little as possible once the flour is added, and don’t be concerned if the dough looks a little crumbly. Toss in the chocolate pieces, and mix only to incorporate.
Turn the dough out onto a work surface, gather it together and divide it in half. Working with one half at a time, shape the dough into logs that are 1 1/2 inches in diameter. Wrap the logs in plastic wrap and refrigerate them for at least 3 hours. (The dough can be refrigerated for up to 3 days or frozen for up to 2 months. If you’ve frozen the dough, you needn’t defrost it before baking — just slice the logs into cookies and bake the cookies 1 minute longer.)
Center a rack in the oven and preheat the oven to 325ºF. Line two baking sheets with parchment or silicone mats.
Using a sharp thin knife, slice the logs into rounds that are 1/2 inch thick. (The rounds are likely to crack as you’re cutting them — don’t be concerned, just squeeze the bits back onto each cookie.) Arrange the rounds on the baking sheets, leaving about 1 inch between them.
Bake the cookies one sheet at a time for 12 minutes — they won’t look done, nor will they be firm, but that’s just the way they should be. Transfer the baking sheet to a cooling rack and cool completely.
Microwave the chopped dark chocolate and coconut oil in 20 second increments until fully melted. Dip each cookie halfway in the chocolate mixture, then set on a cooling rack to harden. Sprinkle immediately with chopped pistachios. Once all your cookies have been dipped, drizzle the remaining chocolate over the cookies.